This invention relates to fiber gratings, and more particularly to a tube-encased pressure-isolated Bragg grating temperature sensor.
It is known in the art of fiber optics that Bragg gratings embedded in an optical fiber may be embedded in a structure and used to sense parameters such as temperature and/or strain of the structure, such as is described in U.S. Pat. No. 4,806,012, entitled xe2x80x9cDistributed, Spatially Resolving Optical Fiber Strain Gauge,xe2x80x9d to Meltz et al., and U.S. Pat. No. 4,996,419, entitled xe2x80x9cDistributed Multiplexed Optical Fiber Bragg Grating Sensor Arrangement,xe2x80x9d to Morey. It is also known that the reflection wavelength xcex of the grating changes with temperature (xcex94xcex/xcex94T) due to the change in refractive index and grating spacing over temperature, such as is described in U.S. Pat. No. 5,042,898, entitled xe2x80x9cIncorporated Bragg Filter Temperature Compensated Optical Waveguide Device,xe2x80x9d to Morey et al.
Also, a fiber Bragg grating may be used in a configuration to measure pressure, such as is discussed in U.S. Pat. No. 6,016,702, entitled xe2x80x9cHigh Sensitivity Fiber Optic Pressure Sensor for Use in Harsh Environments,xe2x80x9d to Robert J. Maron, which is incorporated herein by reference in its entirety. In that case, an optical fiber is attached to a compressible bellows at one location along the fiber and to a rigid structure at a second location along the fiber, with a Bragg grating embedded within the fiber between these two fiber attachment locations and with the grating being in tension. As the bellows is compressed due to an external pressure change, the tension on the fiber grating is reduced, which changes the wavelength of light reflected by the grating.
However, because the grating wavelength also changes with temperature, it is necessary to have an additional grating in thermal proximity to the pressure grating to distinguish between temperature and pressure changes. Typically, the temperature grating is isolated from the pressure signal to provide a temperature measurement independent of pressure, i.e., to temperature-compensate the pressure grating. This requires the temperature grating to be housed in a pressure-isolated chamber. Such pressure-isolated chambers can add cost, complexity, and failure modes to the sensor package.
Moreover, fiber gratings may be used solely as temperature sensors. In that case, a fiber grating by itself will exhibit a wavelength shift due to strains caused by changes in external pressure. Thus, in general, it is desirable to have a fiber grating temperature sensor that measures temperature and is not affected by external pressure changes.
Objects of the present invention include provision of a fiber grating temperature sensor that is isolated from external pressure changes.
According to the present invention, a pressure-isolated fiber optic temperature sensor, comprises: an optical sensing element, having an outer transverse dimension of at least 0.3 mm and having at least one reflective element disposed therein, the reflective element having a reflection wavelength; an optical fiber exiting from at least one axial end of the sensing element; at least a portion of the sensing element having a transverse cross-section which is contiguous and made of substantially the same material; the reflection wavelength changing due to a change in the temperature of the sensing element; and pressure isolating means, fused to an outer surface of the sensing element, for isolating the reflective element from strains due to pressure external to the pressure isolating means, such that the reflection wavelength does not change due to a change in the external pressure.
According further to the present invention, the sensing element comprises: an optical fiber, having at least one reflective element embedded therein; and an inner tube, having the optical fiber and the reflective element encased therein, the inner tube being fused to at least a portion of the fiber. According further to the present invention, the sensing element comprises a large diameter optical waveguide having an outer cladding and an inner core disposed therein and having the reflective element disposed therein.
According further to the present invention, the pressure isolating means comprises: an outer tube, having a first portion fused to a first portion of the inner tube without the reflective element; at least a portion of the outer tube and the sensing element forming a closed chamber; and a second portion of the sensing element with the reflective element disposed therein, extending into the chamber.
According further to the present invention, the sensing element has an optical fiber exiting from the second portion of the sensing element; the outer tube comprises a second portion attached to the fiber; and the fiber passes through the chamber between the second portion of the sensing element and the second portion of the outer tube.
The present invention provides a Bragg grating disposed in an optical sensing element which includes an optical fiber fused to at least a portion of a glass capillary tube (xe2x80x9ctube encased fiber/gratingxe2x80x9d) and/or a large diameter waveguide grating having an optical core and a wide cladding, which is fused within a second outer tube (i.e., a tube-in-a-tube design) which allows the grating to sense temperature changes but is not sensitive to external pressure changes. The element may be made of a glass material, such as silica.
Also, one or more gratings, fiber lasers, or a plurality of fibers or optical cores may be disposed in the sensing element.
The grating(s) or laser(s) may be xe2x80x9cencasedxe2x80x9d in the tube by having the tube fused to the fiber on the grating area and/or on opposite axial ends of the grating area to or a predetermined distance from the grating. The grating(s) or laser(s) may be fused within the tube or partially within or to the outer surface of the tube. Also, the grating(s) or laser(s) may be oriented in any desired direction on the tube, e.g., longitudinally, radially, circumferentially, angled, curved, or other orientations. Also, one or more waveguides and/or the tube encased fiber/gratings may be axially fused to form the sensing element.
Further, the invention may be used as an individual sensor or as a plurality of distributed multiplexed sensors. Also, the invention may be a feed-through design or a non-feed-through design.
The invention may be used in harsh environments, such as in oil and/or gas wells, engines, combustion chambers, etc. For example, the invention may be an all glass sensor capable of operating at high pressures ( greater than 15 kpsi) and high temperatures ( greater than 150xc2x0 C.). The invention will also work equally well in other applications independent of the type of environment.
The foregoing and other objects, features, and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof.